1. Field of the invention
The present invention relates to an audio volume control device, and more particularly to an electronic volume control for an audio device.
2. Description of the Prior Art
Conventional electronic volume controls for electronically controlling the sound level operate by storing the volume control level as a digital value, and directly controlling a digital/analog (D/A) converter according to the stored value.
FIG. 9 is a block diagram of a conventional electronic volume control comprising a volume control register 1 and a D/A converter 5. The volume value Dn applied to volume control register 1 by a host computer or other external input device is latched by the assertion of write clock C. The output value Qn is applied to D/A converter 5 as volume value Dn.
A common D/A converter 5 is shown in FIG. 10. D/A converter 5 comprises plural resistors 20, 21, a switching circuit 22, and an operational amplifier (op amp) 23. D/A converter 5 is a standard current-type D/A converter using common ladder resistors. The value of resistors 20 is suitably set (=R), and the value of resistors 21 is set to twice the resistance of resistor 20 (=2R). Resistors 21 are connected by switching circuit 22 either directly to ground, or indirectly to ground (i.e., virtual ground). Op amp 23 is configured as an inverting amplifier with its non-inverting input tied to ground. It is well-known to those of ordinary skill in this field that an inverting amplifier configured in this way will produce substantially zero volts (i.e., virtual ground) at the inverting input terminal of op amp 23. By setting the resistance of resistors 20, 21 in this way, the current resulting from reference voltage V.sub.REF applied to the signal input terminal is divided at the branching point with half the current flowing to resistor 20 and half to resistor 21. Switching circuit 22 is switched according to the digital data stored in volume control register 1. D0, D1, D2, . . . Dn are individual switches controlled by the various bits of the binary digital data where D0 is controlled by bit 2.sup.n, D1 by bit 2.sup.n-1, D2 by bit 2.sup.n-2, . . . Dn by bit 2.sup.n-n (=2.sup.0). Relative to the current flowing through the switch controlled by the DO bit, the current flowing through the D1-bit controlled switch is 1/2, and the current flowing through the D2-bit controlled switch is 1/4. Thus, the current flowing through each switch is 1/2 the current flowing through the preceding switch.
The output V.sub.OUT of operational amplifier 23 can be defined as EQU V.sub.OUT =k.times.(D/2.sup.n).times.V.sub.REF
where D is the digital value applied by D0, D1, D2, . . . Dn, and k is a proportional constant. If the audio signal is input as reference voltage V.sub.REF, an audio signal for which the volume is adjusted based on the digital value D can be obtained as the output.
When the volume value is input to a conventional electronic volume control as shown in FIG. 9, the control data of the D/A converter also changes simultaneously at the write clock by which volume control register 1 reads the input data. This causes a sharp drop in the envelope of sound wave a at read time t as shown in FIG. 11A. Audible switching noise is thus created, and control of such audio effects as fade-in and fade-out becomes significantly more difficult.